The 1.alpha.,25-dihydroxy vitamin D.sub.2 is the compound having the highest vitamin activity of the vitamin D.sub.2 compounds. Further, the 24-epimer of 1.alpha.,25-dihydroxy vitamin D.sub.2 is very attractive in recent years from the viewpoint of pharmacological activity.
The processes for the preparation of 1.alpha.,25-dihydroxyvitamin D.sub.2 are known by the method of H. F. DeLuca et al disclosed in Bioorganic Chemistry, 13, 158 (1985) and Japanese Patent LOP Publn. No. 501261/1985 (WO84/04527) and by the method of E. G. Baggiolini et al disclosed in J. Org. Chem. 51, 3098 (1986).
However, the former method is not suitable for the synthesis of only the end compound, 1.alpha.,25-dihydroxyvitamin D.sub.2, since a number of isomers are formed and each isomer must be fractionated by high pressure liquid chromatography (HPLC) or the like. Further, the latter method is not satisfactory in an industrial scale, since a large number of process steps are required.
The process for the preparation of the 24-epimer of 1.alpha.,25-dihydroxyvitamin D.sub.2 is known by the method of H. F. DeLuca et al as described in the above references and by the method of H. F. DeLuca et al disclosed in J. Org. Chem. 53, 3450 (1988) and Tetrahedron Letters, 28, 6129 (1987).
However, the former method is not suitable for the same reasons as mentioned above. Further, the latter method is not satisfactory in an industrial scale, since the yield is low and a starting material is expensive.
Now, we have studied the prior art processes for the preparation of 25-dihydroxyvitamin D.sub.2 by irradiation of (22E)-5,7,22-ergostatriene-3.beta.,25-diol followed by isomerization (Tetrahedron Letters, 25, 3347 (1984)) and as a result it was found that a process via new intermediates, (22E)-5,7,22-ergostatriene-1.alpha.,3.beta.,25-triol and its 24-epimer can afford the desired 1.alpha.,25-dihydroxyvitamin D.sub.2 and its 24-epimer in more favorable yields, simpler reaction operation and the like as compared with the above prior art processes.